My copending application describes an improved icebreaking vessel in which pitching motions are induced in the vessel at high amplitude and low frequency. These induced motions create considerable vertical momentum in the bow of the vessel. This momentum is applied to ice ahead of the vessel as the vessel moves along a desired path through ice-covered waters. The vessel breaks ice principally by reliance upon the vertical momentum of the bow, rather than by reliance upon the forward momentum of the vessel, although both forms of momentum cooperate to break ice as the vessel moves under power along its desired path.
The mechanism used to induce these pitching motions are pneumatic mechanisms in which air is applied to the upper extent of suitable pitching tanks to force water from the tanks through large openings through the hull at the lower ends of the tanks; to flood the tanks, the tanks are vented to atmosphere, in the usual case. This alternate flooding and emptying of the pitching tanks produces a fore-and-aft shift in the vessel center of buoyancy, relative to the fixed center of gravity of the vessel, to induce the desired high amplitude, low frequency pitching motions. Thus, the desired motions of the vessel are induced by manipulation of the vessel center of buoyancy relative to the stationary center of gravity of the vessel.
The location of a vessel's center of buoyancy is the centroid of volume of the water displaced by the submerged portions of the hull; the total volume of water displaced by the hull has a weight equal to the weight of the vessel and its contents. The location of the vessel center of gravity is defined by how steel and the like is distributed throughout the vessel during its construction, and how the contents of the vessel are distributed within the vessel. It is apparent that the location of the center of buoyancy is determined by the geometry, i.e., volume distribution, of the submerged portions of the hull. The copending application, therefore, describes pitching tanks which are located wholly below the even-keel load waterline of the vessel; that is, in the copending application, the pitching tanks are located essentially entirely in the submerged volume of the vessel because the tanks are used to manipulate the vessel's center of buoyancy, and buoyancy is a function of submerged volume. Similarly, in other buoyancy shifting systems provided for vessel motion stabilization and the like, such as U.S. Pat. No. 3,689,953, the buoyancy regulation chambers are located wholly below the vessel's even-keel load waterline.
Where a fully submerged chamber is used to regulate the position of a vessel's center of buoyancy, the tank is inherently single-acting in that it can, by flooding or emptying thereof, operate to shift the center of buoyancy back and forth on one side of the normal even-keel position of the center. For example, a normally-empty pitching tank located forwardly of the vessel's normal center of buoyancy and below the even-keel load waterline, when alternately flooded and emptied, is effective to move the center of buoyancy aft of its normal position when the tank is flooded and to move the center to its normal (even-keel) position when the tank is emptied. Thus, a normally-empty bow pitching tank is operable to cause the bow to pitch downwardly from its usual (even-keel) position. Pitching of the bow upwardly from its usual position requires the use of a normally-empty fully submerged pitching tank near the stern of the vessel.
According to the copending application, high amplitude, low frequency pitching motions of the bow of the vessel are desired during icebreaking. The greater the amplitude of bow motion, the greater the momentum of the bow as it moves upwardly or downwardly through its even-keel position; the greater the vertical momentum of the bow, the thicker the ice which can be broken and, usually, the greater the amount, in area, of ice broken. The copending application describes vessels having double-acting bows, i.e., bows which apply vertical bow momentum to break ice during both upstrokes and downstrokes of the bow. In view of these circumstances, the copending application describes vessels having both bow and stern pitching tanks which are operated out of phase with each other to produce both upward and downward excursions of the bow from its normal position, thereby producing high overall amplitudes of pitching motion to generate large amounts of vertical momentum in the bow.
Where an induced pitching icebreaker has a bow of conventional single-acting configuration, best performance of the vessel in ice is obtained where the induced motions of the bow are upwardly from the normal position of the bow.
Preferably the pitching tanks are of the normally-empty type. That is, during operation of the vessel in ice-free waters, the tanks are empty and their discharge ports are closed for maximum statical stability and maximum propulsive efficiency of the vessel.
These factors are at odds with the fact that, for greatest volumetric efficiency, a stern pitching tank should be located as far aft in the vessel as possible, and the fact that in most vessels, space, especially pitching tank space, is not readily available in the stern portion. Usually, there is considerably more space available near the bow for a pitching tank of given volume than near the stern.
It will be seen that a need exists, in the context of induced motion vessels for icebreaking and the like, for a double-acting motion inducing arrangement which can be located away from the stern of the vessel and which can be operated to produce vessel motions in both directions from the even-keel position of the vessel.